EKCO U122 - Restore or spares?

[DonaldStott]

Originally Posted by DonaldStott

1) ... I also note that V4 Pin 4 is used as an anchor tag linking C22 and R18. We've all done the DAC90A UL41 Pin 4 Mods so I'm assuming the same would apply here?

ERRATUM: Apologies - of course this should read C26 and R21.

[DonaldStott]

This U122 has a couple of capacitors (50pF and 100pF) which are of a type I've never encountered before - it's the small, cream coloured cap in the middle with terracotta ends.



They look ceramic in nature but don't know about their reliability.

And of course they are impossible to test in circuit!

[ms660]

Those capacitors were quite common, used in TV's as well, they were very reliable back in the day and might still be, I don't remember finding a faulty one.

Lawrence.

[ms660]

Originally Posted by DonaldStott

And of course they are impossible to test in circuit!

C8...Resistance (leakage) if you look the schematic you'll see that there should be no external DC path to give a false reading.

C8...Value, if you can snip the chassis end of R4's wire you should be able to measure the value of C8 (as near as damn it) with a suitable meter.

Lawrence.

[DonaldStott]

Originally Posted by DonaldStott

... I also note that V4 Pin 4 is used as an anchor tag linking C22 and R18. We've all done the DAC90A UL41 Pin 4 Mods so I'm assuming the same would apply here?

If V4 Pin 4 is not used as an anchor tag then the connection between C22 and R18 would be floating but the leads would be well insulated?

Similarly V4 Pin 3 is connected to the centre of the valve holder and to an adjacent chassis tag - again from my DAC90A experiences I have always thought that there should be no connections to Pins 3 and 4 of the UL41?

I've removed the Phantom's 150R, BY127, polo-mints and associated wiring AND am trying to get my head around where the connections to the Dropper should be when I reinstate a UY41.

[ms660]

Originally Posted by DonaldStott

If V4 Pin 4 is not used as an anchor tag then the connection between C22 and R18 would be floating but the leads would be well insulated?

Similarly V4 Pin 3 is connected to the centre of the valve holder and to an adjacent chassis tag - again from my DAC90A experiences I have always thought that there should be no connections to Pins 3 and 4 of the UL41?

I've removed the Phantom's 150R, BY127, polo-mints and associated wiring AND am trying to get my head around where the connections to the Dropper should be when I reinstate a UY41.

Pin 3 of the UL41 valve is connected to the cathode as is pin 7, so it's no problem if it's connected to the chassis, I can't quite tell from the photo's but if it's the only cathode connection to chassis then it should be left as is for sure......

In the Bush DAC90a pin 3 of the UL41 is the cathodes connection to chassis via it's 150 Ohm cathode bias resistor, the Trader sheet for the Bush DAC90a shows the pin 3 and pin 7 connections to the cathode in their valve base diagram for the UL41.

Originally Posted by DonaldStott

I've removed the Phantom's 150R, BY127, polo-mints and associated wiring AND am trying to get my head around where the connections to the Dropper should be when I reinstate a UY41.

The anode of the UY41 should be connected to the top of the dropper which is the anode side of the 70 Ohm surge limiter section, the rest of the dropper connections should remain as they are so far as I can make out from your drawing.

Lawrence.

[DonaldStott]

Almost at the stage where I have undone all the previous 'changes' and can actually make a start on restoration of this set.

As I said in a previous Post "It's especially difficult with sets such as this which have been 'got at' in the past as you are trying to mentally undo what was in the mind of the 'Phantom Twiddler'!"

Looking carefully at the circuit diagram I am still at a loss as to why there is a connection from the +ve side of C28 to the top of the Dropper? As far as I can make out there should only be two connections to the +ve side of C28, one to Pin 7 (Cathode) of V5 (UY41) and one to T1 (Output Transformer)?

[ms660]

Originally Posted by DonaldStott

Looking carefully at the circuit diagram I am still at a loss as to why there is a connection from the +ve side of C28 to the top of the Dropper? As far as I can make out there should only be two connections to the +ve side of C28, one to Pin 7 (Cathode) of V5 (UY41) and one to T1 (Output Transformer)?

All that should be connected to the top of the dropper in the dropper diagram you drew should be the anode of the rectifier, as per the schematic.

Lawrence.

[David G4EBT]

Personally, with a set like this which has so evidently been bodged is a rather atrocious manner, I just remove all of the 'bodgery' and rebuild it as it originally was. With this set, if I felt it worth restoring (that ship has already set sail), the first thing to go would be the silicon rectifier, to be replaced with a UY41, and secondly, I'd scrap the mains dropper and replace it with a new one.

I haven't followed the thread closely, but as far as I can tell, it's Trader Sheet 995 which is being used. If so, I find the circuit layout messy and difficult to follow as compared the the Maker's own service sheet, which to my eyes, is much clearer and easier to follow.

Much discussion has been around the mains dropper and the RS circular dropper sections.

Whilst maybe the original could maybe left in place, it's in a poor state, and for me, it would have to go. As to a replacement, one of the taps on the original is for 115V mains so isn't needed on a replacement dropper. In the now closed 'dropper wanted' thread, Gabriel at post #17 suggested a replacement 1K high wattage wire-wound resistor with adjustable taps which can cheaply be obtained. They normally only have one tap but a second clip could easily be fashioned to reduce the 1,000 Ohms down to the 888 total Ohms called for, though 1,000 Oms is only 12% higher, so could no doubt be used without the second tap to drop 112 Ohms. I doubt it would make and discernible difference in performance and the radio would be a bit less stressed.

To make things a bit clearer, I've attached a sketch showing the original dropper and a sketch alongside of a suggested universal replacement.

I've also attached a picture of a 1,000 Ohm 50 Watt dropper similar to the one that Gabriel suggested, which I obtained some years ago from China and used to good effect (on a different radio). The Wattage needed for the U122 probably wouldn't need to be that high - I haven't tried to calculate it - but the 50 Watt one I used was only 8cms (3.25") tall and 2cms diam, so I guess it would fit and if the dissipation is well below 50 Watts, that would be all to the good. (The mounting brackets would of course need to be removed and the dropper bolted as was the original).

Just my thoughts.

Hope that might help a bit.

[Radio Wrangler]

Originally Posted by ms660

I might be wrong but looking at the transformer and the data sheet photo's it looks like the primary hasn't been connected for optimum anode load impedance.....9k as opposed to 3k.

I noticed that the output winding with its tap is treated as if the impedance to be driven by the full winding is equal to the sum of the impedances if each section is used alone. Not strictly right but close enough an approximation in this case, I suppose.

David

[ms660]

Originally Posted by Radio Wrangler

I noticed that the output winding with its tap is treated as if the impedance to be driven by the full winding is equal to the sum of the impedances if each section is used alone. Not strictly right but close enough an approximation in this case, I suppose

Hum cancelling transformer article from my reference book here, compare the primary winding resistances for this type of transformer typically found in other receivers to the receiver in question.

The 3k optimum load given in the valve data is for Va = Vg2, in the receiver this won't be the case.

Lawrence.

[ms660]

Originally Posted by David G4EBT

To make things a bit clearer, I've attached a sketch showing the original dropper and a sketch alongside of a suggested universal replacement.

Ref. the drawing of the original.

There are in fact 6 tags on the original dropper, it might not be that evident from the manufactures service information but the last section at the top of the dropper is a 70 Ohm section, this is clearly shown on the Trader sheet where the 70 Ohm in question has a specific circuit reference (R23) both in the schematic and on the Trader sheets plan view of the chassis where it's marked as R23 with its two connecting tags on the top section of the dropper.

Lawrence.

[DonaldStott]

Thanks David, Lawrence and David

I've attached an updated Dropper diagram showing the existing tags, connections and resistance values - hope this helps to clarify matters.



In relation to the Output Transformer impedance I'm assuming that connections to Tags 2 and 3 giving an optimum 3k is still preferred?

[ms660]

Originally Posted by DonaldStott

Thanks David, Lawrence and David

I've attached an updated Dropper diagram showing the existing tags, connections and resistance values - hope this helps to clarify matters.

Attachment 228995

In relation to the Output Transformer impedance I'm assuming that connections to Tags 2 and 3 giving an optimum 3k is still preferred?

Attachment 228997

Obliterate the 128 Ohms between tags 2 and 3 and change the 680 Ohms to 670 Ohms then it will be as per the schematic and easier to reference to.

As said, the 3k load in the valve data is given for when Va = Vg2, Va and Vg2 shouldn't be the same in your receiver so use the taps that work the best.

Lawrence.

[DonaldStott]

Originally Posted by ms660

Obliterate the 128 Ohms between tags 2 and 3 and change the 680 Ohms to 670 Ohms then it will be as per the schematic and easier to reference to.

Not sure what you mean by "Obliterate the 128 Ohms between tags 2 and 3 ..." - sounds a bit drastic!

Originally Posted by ms660

As said, the 3k load in the valve data is given for when Va = Vg2, Va and Vg2 shouldn't be the same in your receiver so use the taps that work the best.

Understood - the connections I've made to the Output Transformer Primary are to tags 2 and 3 which, according to the Radiospares Data Sheet, gives an Optimum Load of 3,000 ohms. (DC resistance across tags 2 and 3 is 297 ohms).

As you say all good and well when Va = Vg2 but the valve data for this set lists Va = 213V and Vg2 = 76V.

If and when this set is ever fired up we'll see what the valve pin voltages are telling us and adjust the tags accordingly.

[ms660]

Originally Posted by DonaldStott

Not sure what you mean by "Obliterate the 128 Ohms between tags 2 and 3 ..." - sounds a bit drastic!

Understood - the connections I've made to the Output Transformer Primary are to tags 2 and 3 which, according to the Radiospares Data Sheet, gives an Optimum Load of 3,000 ohms. (DC resistance across tags 2 and 3 is 297 ohms).

As you say all good and well when Va = Vg2 but the valve data for this set lists Va = 213V and Vg2 = 76V.

If and when this set is ever fired up we'll see what the valve pin voltages are telling us and adjust the tags accordingly.

I would have labelled the dropper sections with their circuit references and resistance values given in the Trader sheet and put their measured values alongside.

Vg2 will be less than Va unless it's connection has been frigged or there's a fault.

Lawrence.

[DonaldStott]

Another mystery capacitor - the metal cased one in the middle of the picture with the band around it:-



This is C20 0.01uF (A.F. Coupling) in the Trader and is connected at one end to Pin 3 of V3 (UBC41) and at the other end to R11. The band around the middle is connected to R12 which then also connects to Pin 3 of V3.

The existing metal cased capacitor also appears to have different coloured ends, black and terracotta, implying that it is directional?

As this is a coupling capacitor it will be replaced by a modern 0.01uF 630V polyester capacitor (BVWS) but I am unsure how to connect it up?

[Radio Wrangler]

Looks like a TCC 'Metalmite' - which I've not found to be too good these days.
Paper dielectric, moisture diffuses in through the rubber bungs and they go leaky.

David

[DonaldStott]

Thanks David - it does appear to be a TCC 'Metalmite', indeed the same as C24, also A.F. Coupling.

Further research i.e. a quick 'Search this website', led to this explanatory Thread: -

https://www.vintage-radio.net/forum/...d.php?t=104856

Still confused though by the three connections to this capacitor, one at each end and one to the band in the middle?

With the replacement I'm just going to follow the circuit diagram: -

[ms660]

It's a shielded capacitor.... when the case is connected to chassis, it shields the actual capacitor inside from stray pickup from unwanted sources.

The high value (10meg) of the resistor in the grid/cathode circuit is used because the triodes bias is obtained by grid current biasing instead of using cathode bias, in some receivers the resistors value was higher still, 22meg being used in some designs.

Black at one end or a black band near one end usually indicates the end that the capacitors outer foil is connected to.

I think some folks on the forum have shielded an ordinary replacement by various means, copper tube etc.

EDIT: post crossed.

Lawrence.